The prior art has already described multilayer petrol tanks. EP 834,415 describes structures comprising: polyethylene/binder/EVOH/binder/polyethylene.
The binder is a maleic-anhydride-grafted polyethylene having an MFI of 0.1 to 3 and a relative density between 0.920 and 0.930 and it contains 2 to 40% by weight of material insoluble in n-decane at 90° C. It is explained that the grafted polyethylene is dissolved in n-decane at 140° C. and cooled to 90° C., at which temperature products precipitate; it is then filtered and the insoluble content is the percentage by weight which precipitates and is collected by filtration at 90° C.
If the content is between 2 and 40%, the binder has good petrol resistance.
No example shows such a polymer. The text specifies that the binder is in fact a blend to 2 to 30 parts of a grafted polyethylene having a relative density between 0.930 and 0.980 and of 70 to 98 parts of an ungrafted polyethylene having a relative density between 0.910 and 0.940.
An attempt has been made to prepare binders and the corresponding structures in accordance with this teaching. It has found that the binders were not reproducible, that is to say the choice of the product by its relative density is not a sufficient indication. It has also been found that these criteria were not sufficient for the binder to withstand petrol.